This #$%^&*@ car! long post

[bsell]

UPDATE
There is a spark knock when the throttle is snapped open as well.
What are your automatic M50s like off idle when the throttle is snapped open from idle??

Spark knock with no chip and good fuel? Sounds like crank sensor signal or engine computer freaking out.? Or is the crank sensor in the wrong position for some reason (road crap knocking it around)? Seems a reach... After typing the below, I got to wondering if you have carbon build up in the combustion chambers that might require some Seafoam-type action? That would explain the spark knock on throttle snaps.

Didn't I read on here the other week about somebody cursing their coolant temp sensor for causing a driveability problem like yours. They did the very same things you have done and were kicking themselves after it all came down to a sub $20 part.

My 95 525 auto responds instantly from idle snaps, but not as strongly as when snapped at higher rpm's.

I would love to see the vacuum and fuel pressure gauges during your stumbles. That might be very illuminating.

Brian

[Ross]

Thanks for mentioning "good" gas, the tank was near bone dry and I added fuel from the can for the lawn tractor(regular)so it wouldn't run out. This never ocurred before so I'm guessing the knock is a red herring and due to the gas.
Tomorrow I'm going to check all the sensors I can find a spec for.
I haven't given the coolant temp sensor much thought, perhaps I should.
Carbon is a real possibility as the GF drove this car and she is not one to drive it hard. I whooped it a bit today, no smoke to indicate a big build-up.
The ICV keeps coming to mind, it's clean and rattles open and shut if you shake it. Some better test for this is in order I think.
I'm on a mission now. Damn the cost!

[Brandon J]

Another thing to add to your list of things to check is the vanos. I am assuming you already checked for any fault codes too. If you sense a knock, then the computer will sense this and dial back timing, hence the bogging feeling. Fuel injectors can get dirty or go bad, fuel filter needs to be changed, spark plugs are the wrong ones, if aftermarket chip - I know of a few that have gone bad, get good premium gas, cracked intake manifold, etc.

Good Luck.

[Ross]

Thanks Brandon,
There is regular gas in it right now. The injectors have been serviced out of the car, one is new. No codes. Fuel pressure and starvation I've ruled out because it's a low speed issue.
I removed the intake today. inspected it, checked the ICV resistance and found it to way off the BENTLEY spec. A new one provided the same readings as mine so I didn't change that.
Since I reused the intake gaskets it seems I now DO have a vacuum leak because it's surging now. Gaskets on order.
Plugs are new and correct, the coils were also swapped with known good.
The coolant temp sensor is good as is the intake air temp sensor.
I'll bet this ****er has a vacuum leak somewhere(now for sure at the intake) but it wouldn't turn up no matter how much stuff I spray around the possible areas. Some hoses are soft but none have a detectable leak.
I will read up on VANOS. Anyone else have vanos trouble that manifests itself as a low speed/ off idle stutter/stumble??
It was high 90s here today, man am I sick of this.

[Ferret]

Thanks Brandon,
There is regular gas in it right now. The injectors have been serviced out of the car, one is new. No codes. Fuel pressure and starvation I've ruled out because it's a low speed issue.
I removed the intake today. inspected it, checked the ICV resistance and found it to way off the BENTLEY spec. A new one provided the same readings as mine so I didn't change that.
Since I reused the intake gaskets it seems I now DO have a vacuum leak because it's surging now. Gaskets on order.
Plugs are new and correct, the coils were also swapped with known good.
The coolant temp sensor is good as is the intake air temp sensor.
I'll bet this ****er has a vacuum leak somewhere(now for sure at the intake) but it wouldn't turn up no matter how much stuff I spray around the possible areas. Some hoses are soft but none have a detectable leak.
I will read up on VANOS. Anyone else have vanos trouble that manifests itself as a low speed/ off idle stutter/stumble??
It was high 90s here today, man am I sick of this.

I was reading the other day that if something trips the ECU into default mode in VANOS cars, the ECU will reset the vanos unit back to a default timing. In the later M50 class engines out of the e39 - if the engine ran with the vanos defaulted for more than a few weeks it'd need a rebuild as the unit would stick.

I've no idea if this relevant in the M50 unit and whether the o2 sensor code causes it to default time or not?

Despite you've cleaned the MAF, have you tried swapping in a good one from one of your other cars? Hesitation off idle is almost always a lean mixture fault - which you probably know already. This might sound daft, but have you had a look at the gaskets on the ICV to manifold hose?

[Ross]

Thanks Ferret,
Ya, I tried the MAF from my 530(same#) it was the same.
There is an o-ring on the intake for the IAC's hose connector that I've checked.
The Vanos thing is interesting though, I need to read up. This was running poorly a long time, since I seldom drove this car and the GF doesn't complain the problem was neglected. I hope the Vanos isn't F'ed.
I'm about to employ the advice suggested in your sig.
Ross

[Ferret]

I'm about to employ the advice suggested in your sig.

Ha!

Other things to check are the wiring harness upto the vanos unit itself - I'd have a quick run around with a can of WD40 and clean all the connectors up.

Doesnt the VANOS mechanical unit have a built in oil-filter inside the housing that can get fouled up and cause the unit to seize again am I crossing it over with an e39? I've heard horror stories about these as well.

It might be worth pulling the vanos unit off the front of the engine and having a look to see how well it articulates and/or is gunked up. If the engine's been running like crap for a while the oil will probably have been depositing crap all over the engine.

You could possibly try ripping the vanos unit off one of your other cars and transplanting it in place and see if it makes a difference.

[Brandon J]

I wouldn't suggest diving into your vanos if you don't know what you are doing or how to set it. Rule the cheaper stuff out first. Do you have the stock chip in there?

Before my S38 M5 transplant I had an M50tu with M3 cams, S50 valve springs, injectors, 2 custom chips, larger MAF, lightweight flywheel, 3.46 diff, custom intake, etc. I had a new vanos put in after it was making noise and sticking. Do some more research and you might have to take it in to a professional.

One thing about the fuel pump and filter, fuel pressure can build up in the lines on idle. Once that pressure is released, a clogged filter or bad pump will need more time again to build the necessary pressure. Also, if a coil did go bad, sometimes it can take the ecu with it. Seen it happen several times. With a bad coil, bad ecu, fuel filter/pump, or vacuum leak can all give the same symptoms. What are your MPGs? What does the exhaust smell like? Any fuel smell from the engine bay? Does it get better once warmed up? When colder outside? How do the spark plugs look? Any transmission hiccups ever?

Also, at times when a tank goes almost empty, the pump picks up sediment at the bottom of the tank. It clogs the fuel filter. I have seen this happen a few times.

Tip: If you decide to change the fuel filter, take a sharpie and write the date and mileage on it before installing.

[Ross]

Brando,
I've swapped the coils and no improvement. Fuel filter 17k ago. Plugs were fine except from the cylinder with the bad injector, all since replaced with new Bosch.
I'm assuming the stock chip as this was never to knowledge owned by anyone that would modify it.
Regarding the fuel system, I haven't checked absolut pressure as my gauge is set for a schrader valve(that BMW can't seem to provide on a $40,000 car), I'm going to rig a tee when I redo the intake gaskets.
The exhaust smells right now that the injector issue is taken care of.
Funny that you mention the transmission. Sometimes I's swear the stutter is the the torque converter. But it still has the off idle bog even with a free rev when the throttle is snapped open.
MPG was ~17 according to the OBC, admittedly poor even for in town traffic, I think her avg. MPH was only in the twenties.
No fuel smell under the hood and the canister is fresh.
As far as going to a professional, two have looked at it, one DEALER tech failed to recognize the problem and after I explained to the other all that has been tried so far he shrugged and said "I don't know, sell it." Old afshioned diagnostic skills are hard to come by. There are currently no codes and you have seen the litany of parts exchanged.
It's gonna be something simple, I just haven't found it yet.
I'm not afraid of the vanos but would certainly prefer not to fool with it.
I'm going to buy all the hoses, seals, gaskets related to the intake and see what happens. I have looked numerous times for a vacuum leak and come up empty, perhaps a few undetectable ones conspiring.
You're a Chicago guy, right? How would you like to take it for a spin?
Bill R, Winfred, Kalevera what have you guys got to say?
EDIT BTW With the intake off I was able to see the valves of course, no inordinate deposits in the intake tract.

[Ross]

UPDATE for those following my misery
The surge I attributed to leaking reused intake gaskets was due to the hose at the carbon canister purge valve being disconnected. Found this while examining all the hoses for what size to order.
Problem is now far better after sealing up potential but undetected leaks at the fitting for the pressure regulator hose fitting on the intake and intake air temp sensor and of course the disconnected hose.
I've looked at nearly everything yet overlooked something.